However, these gold coatings, in a thickness of between 2.5 μm and 5 μm, are disruptive to the soldering process, especially since the gold can form intermetallic, brittle phases with the applied soldering tin, making the soldered join more brittle. Therefore, the user or purchaser is forced to remove the gold coating before soldering the outer contacts onto a higher-level printed circuit board. On the other hand, the outer terminals or outer contacts, which predominantly comprise copper or copper alloys, of a semiconductor device without the gold protection would have a very limited storage life, since the copper becomes tarnished, and the tarnished layer of copper oxide and copper sulfide cannot be reliably wetted.
Moreover, gold coatings are used on components of the semiconductor device on which the back surfaces of semiconductor chips are to be secured. In these cases, gold layers with a thickness of from 2.5 μm to 5 μm have likewise proven suitable for achieving a reliable eutectic fused join between the gold coating and the silicon of the back surface of the semiconductor chip. Furthermore, in a semiconductor device gold coatings on contact terminal surfaces of wiring structures in a thickness of from 2.5 μm to 5 μm are used to attach bonding wires via eutectic fusion with the aluminum of the bonding wires.
Internal flat conductors in conventional devices also have a gold coating of this type with a thickness of over 2.5 μm, so as to ensure that the bonding wire coming from the contact surfaces of the semiconductor chip can be bonded onto contact terminal surfaces of a flat conductor. With regard to the thickness of the layers, it is attempted to save on production costs by applying the gold selectively to the locations of the components at which a gold coating is required. However, it has been found that alignment and selective application of a gold coating of this type forces the manufacturing costs upwards.